Multichannel telemetric system



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D. B. HARRIS MULTICHANNEL TELEMETRIC SYSTEM Filed Jan. 20

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May 19, 1953 z# UNA/@wa Patented May 19, 1953 mais Hammam sita. ma. toolli'la Ridioom'mny; Ceder Rapids, Iowa a corporation of ,Iowa

nplicetienjanuary 20, No.. 139.7`1,

This invention iel 1 to telemetric and moreespeciallyto systemsI- or controllihg the angular-setting of a slate shaft from a master shaft by means of an electrical transmission link. v

A principal object of the invention relates to aV novel method for independently' controlling e multiplicity of slave shafts from rnsectve master shafts using a, single electricallnk between all the master Shafts: and all the 'slave shafts.

neemt principal :moetk isk te provide me 'proved and simplified' multi-enamel synohr system. i

Another object is to provide a novel multisynomo system 'employing s; multiplicity of mster synehrns and eorrespond'mg stave synehtos which ate interconnected bja single elctrica! titanemission channel, and wherein the slave demos tn mselves 'niet as frequency selective deinem.. la rs. A

A further object. relates to notte) multiplexing system fori contraints'. a.. mumpnciuy o! slave shafts,

A feature of the invention relatesA to i multisyhehro system having? a p lliiiity o! msteljeyitehi-os each excited by sirioa'nd cosine functions of respective harmonics of a master oscilletci' frequency to produce in respective rotor outputs of the master synchrosf, respective phase modulated carriers. The slave synehros have their rotors au energized in paradisi by the received carriers; and th slavesynchros themselves act as frequencyseleotive phase demoduletors,

Another featurev relates to novel' multioitaitf nel phase modulation transmitter employing a pluzlity of synchros Another feature :falaises to a) novel multiohom nel phase modulationreeeiving on-'armementployimgr ai` plurality etsillu'ihflfot s y A further feature elates' Athe novel oiganizftio` n arrangement 'arid relative location o nd interconnection of partswhieh. cooper/ete to' otovis'je sn improved multiplex tiileiietrie,jsysieiyt-.

Other features and edvantagesjiot particularly enumerated?, Wlllbeappujent site; ai, considraeen sf the following downed. descptmn and the appeded claims.

"The single fige of thel drawing showgirl whfritic', firms diagram ,and 66k diagram form a typical leifnbcdineiit ci' the i-iivetion.

It can be shown that ttvocarrier freqiiencies* iii quadrature relation, for example Sine and cosine,l retrospectively modultd by scips; (chtite- 119i 'niitted spectrr'rieari be expressed es iollows:

ela-.remmtts meen-ci son fio whe o is' theiricsntnoufs voltige i moet@ 151? smdiiiiid We@ .Pi-Mt) ,iS @A @www :riemen identified ,as tong essoeisted with e ea.-

vsin@ carrie; bysubscript c, n is ahy iriteger, w

isgtfhe seguier frequency of, a fundamental earler wave, and Frmd) is a modulating function identi- "as being associated with a sine carrier-by' submit?? b. then.. Order tof demduate the e9- rrirs, itis-only necessary tqmultiplxhy wt and, inesrate, to 0mm s demoduxswr while, if it. is desired to demodulate one of the sine carriers, the complex wave is multipleclhy sin mut, and the result is integrated to obtain i 7i depends. on the. characteristics of th dev-ice. f cordaxioe with onefeatlxre o the preaeht on., the lnuodtijliitingy functions; ere Vthe mmf angular relations for the purposel 'o1 nroe", onde. @modulating .frequency wwbase i, AW was whienrenresenthe or tation oi. a master short. b y making gies ot the function Petit) and the function liceos with respectto am original loting-signal. and by selection of: suitable form io'xxthese modulating functions Fmt) 6nd Fln t satisfactory lilrxoeiev modulation of 'e carrielyfreduew! can beachievedwhi-le employing conventional kamtlitmcie modulation teolfmidues. tion of Equation 1 that themanulatiriaiunctions Finiti und had) musttin order to specify fully a nnodiilatedv wave, havevv a form of the following geneil nature:

mit) :tsm-Gun other.

voltage being transmitted. Then the modulating functions become Fan) :'yTEknEmn COS SnZEon COS 1m (8) and I ei=l2Enn cos qm cos nwt-i-EEM sin on sin nwt] then the original phase-dependent modulating age, emu.

,. of the device employed to set up the modulating It remains, of course, to determine just what sort of modulating function we need to create in .ordento produce, through thisV process,.a phase modulated wave in which the phase deviation is a function of the fundamental modulating volt- It is immediately seen that the primaryrrequirement is expressed by the equation,

(fm :kennt (15) where 1c is a constant depending on the structure function. Since the modulating waves required functions can be recovered at the receiving terminal by multiplying by a wave having the same frequency and phase as the carrier, and integrating, in the manner of Equations 2 and 3, which are here written in the following form:

and

21.- L?? 0 ,/wersin www@ (12) These equations have a particular application,

`as will be demonstrated later, in the case of data transmission, where it is desired to 'transmit shaft position information from one point to'an- Here, the independent variable, pn :is actually the angle of rotation of a shaft, and the 'modulating waves Emu cos on and Emu'sin on are obtained directly from synchro generators coupled to the shaft. Equations 1l and 12 therefore l'provide the basis for the operation of a simplified multi-channel synchro system, in which a number of separate shaft positions may be transmitted simultaneously over a common channel on separate carrier frequencies. A broader application of this principle tothe problem of phase modulation in general is 'discovered by expanding Equation l0 in accordance 'with the trigonometric identity,

cos A cos B-l-sin A sin B=cos (AB) ji-(13) which leads immediately to the equation,

ef=l2Em cos on cos mvt-l-EEnn sin on sin mut] :ZEW cos (mut-4m) This equation states in effect that if a carrier wave is amplitude modulated by a modulating function in which the angle is the independent variable, to kproduce a firstlmodulation'product; and if the same carrier wave inquadrature is modulation product; then the sum of the first `and second modulation productsis a phasemodulatedwave The importance of the relationship .resides in the fact that `the actual modulation Vprocess in which the modulating function is impressed on the carrier involves amplitude modulation only. Conventional and` simple amplitude modulators may therefore be employed inthe v760 'amplitude modulated by the'same modulating function also in quadrature, to form a second radio-frequency section to produce the required phase modulation. The preparation of the nec- `essary modulating functions to be applied Eto the amplitude modulators maybe carried, outyat 'modulating frequencies.`

' the output 'of this device are speciiied by Equations 6 and 7, we immediately have, by substitution of (15) in these equations, the requirement that the output of the device, which might be calleda phase converter must be GMU) :Emu COS kmn (16) 'and 'A fUnder these conditions, the equation of the modulated wave becomes Gland) :Emu Sin kemn which is the equation of a phase modulatedhwavc vhaving a phase deviationproportional to the modulating voltage.

summarizing the requirements of this type of phase-modulation system, it'is seen that the first'4 step-'is the generation, by means of phase converters, of two modulating waves, the -in- .-sta-ntaneous values'of which are proportional respectively to the cosine and to the sine of` an angle which in turn is proportional to the modulating voltage. These two modulating waves are usedrespectvely to amplitude modulate a cosine carrier and a sine carrier, and the outputs of the amplitude modulators are added. The result is a phase modulatedwave in which the phase deviation is proportional to the original modulating voltage.

The accompanying drawing is a schematic dia- V'gram of a multiplex ormulti-chahnel synchro system basedA upon the principles of Equations .10, l1 and l2, which is capable of transmitting more thanone hundred shaft positionsover two 3,000-cycle widelines, or radio channels, with- .out carrier preselection at the receiving stations,

and by using the synchros themselves as selective demodulators. Merely for simplicity in the drawing, there are shown two master 0r transmitting synchros I0, I I, and two corresponding slave or receiving-synchros I 2; I3. Since such synchros are well-known in the art, detailed de- -scription thereof is not required herein. For a .detailed description of a typical structure of such a synchro, reference may be had to volume 17 --ofRadiation Laboratory Series, entitled- Components Handbook, chapter 10, pages 340 et -seq.,paragraphs 10.12 and 10.13', published by McGraw-Hill Book Company; Inc., New York,

.New York. Itwill be understood, of course, that ,theterm synchro is used in a generic sense to cover so-called rotar-y transformers, such for example as those identified inthe-.trade as Sel- ,syn, Autosyn, etc. Thus the synchro IU'con- Nsists of two stator windings I4,A I5, which are mounted in phase quadrature` relation. Cooperating. with thetwo stator windings is a rotor winding I6 whose angular orientation with res'pec't lto the stator windings can be varied; Thus the. winding `I6 can beattached to a manually 5 rotatable shaft I1 forvarying its angular setting.

Likewise, each .of `the slave synchrosvfor example synchro I2, comprises a pair of stator windings I8, I 0, mounted*v in phase quadrature relation, and cooperating with a respective rotor winding twhichis attached toa shaft 2i whose'setting is to' be lcontrolled from' thev shaft Il; Merely for simplicity'inexplanation, theV stator windings le and Iii will be referred to herein as the vertical windings, and the stator windings I5 and la will' be referredv to' herein as the horizontal windings. Byenergization of the'stator windings, for example windings Ill and I5, according to theinvention, any particular angular setting of shaft l1 willA result' in a voltage being transmitted over the line or transmission channel-22 which is commonto'all;thesynchros. Likewise, by energization oi the stator windings` of the slave synchros, according to the invention, these tran'smitted'voltages set up corresponding torques in the shafts of the slave synchros, whichtliereupon assume the same angular setting as the corresponding master synchro shaft. Merely for simplicity, the master synchros will be referred to herein ats-transmitting'synohros, andthe slavey synchros will be `referred to as receiving synch-ros.

A master oscillator of anywell-known type, controls the system. For a detailed description of a typical master oscillator, reference may be had to Ultra-High Frequency Techniques by Brainerd, Koehler, Reich, and Woodruff, twenty- 'First print, page' 206; published by D. Van Nostrand Company, I'ncr, 250 Fourth Avenue, New York, N, Y. It will be understood, ofcourse, that any other well-known' form of sustained oscillator-generator oivhighly stabilized frequency may be employed. The sine waves from the oscillator 23 are then converted into corresponding sharp or triggering pulses by any well-known pulse-forming network 2li, such for example as disclosed page 178 of above-notedl publication. It will be understood, of course, that the invention is not limited to any particular way of producing the triggering pulses, and any other well-known triggering pulse source of master frequency may be employed, such for example as disclosed` in chapter e of the above-identied publication. The triggering pulses are then applied to the input of a multivibrator 25 of any well-known type, preferably of the driven type, such for example as disclosed in Principles of Radar. by members of the staff of the Radar School, Massachusetts Institute of Technology, second edition., chapter II, pages 2--50 to 2i-53, published by llllcGraw--li-ll Book Company, Inc., New York and London. In accordance with the well-known operation of multivibrators, there is produced at the output terminals of multivibrator 25 non-sinusoidal wave which is rich in harmonicsl at a fundamental carrier frequency, which for example may be 25 cycles per second.

harmoni'c'all'y related carriers are then applied in parallel to a series of synchronized oscillators 2li, 2T, there being one of such oscillators for each of the corresponding transmitting synchros. For a detailedy disclosure of such a typical oscillator, reference may be had to the aboveidentified publication Principles of Radar, chapter VII, page 7-72. The various oscillators 26, 2l, etc., are tuned to different harmonics of the iundanlental frequency from multivibrator 125. For example, the oscillator 26 may be tuned to the sixteenth harmonic, namely 4.00 cycles per second; the next oscillator 21 `may be tuned to the next succeeding harmonic, so that if 100 synchronizeol oscillators are employed, they may be respectively tuned to a particular frequency in the range, for example, between 400 and 3,000

6, cycles, per second, corresponding toy the range from the sixteenth to the one-hundred and twentieth harmonic of the fundamental frequency from multivibrator 252 The output carrier from each synchronized oscillator, for example oscillator 26, is applied directly across the horizontal stator winding I5` of the associated transmitting; synchro, while the oscillator carrier output is, appliedY to the associated vertical stator Winding I4 through a phase-shifting network 28 which alters the'y phase of the carrier by 90 degrees. Therefore' the voltages across the horizontal stator I5.: and the vertical stator I4 are represented respectivelyy by plus cos nwt and plus sin nuit., 'Fliese sine. and cosine carriers induce corresponding volta-ges in the rotor winding I6 which are thereupon impressed upon the transmission channel 22. At the same time, the sine and cosine carriers from the remaining vtransmitting synchros I I, etc., are also impressed upon the transmission channel 22.

Alli these sine and cosine. carriers are impressed in parallel upon the rotor windings 20, etc., of the receiving: synchros; I2, i3, etc. The master frequency pulses: from: the oscillator 23 are also transmitted over a control or pilot channel 29, and are applied to a multivibrator 311|l whichr may be similar to multivibrator' 25. The output of multivibrator 30 is applied in parallel to a series of synchronized oscillators 3|, 32, etc., similar and tuned. respectively to thev corresponding oscillators 26h, 2:1, etc. The. output of each oscillator 31 32, etc., is applied directly tov the vertical stator coil I8 of the associated receiving synchro, while the horizontal statorwinding I 9 of the associated synchroy is energized from itsr synchron-ized oscillator through a phase-shifting networky 33/ which displaces the phase by 90 degrees. Thus the voltage across winding I 8 is represented as minus cos nuit, and the voltage across the winding I9 is plus sin mut. As a reoii this energizationof the stator windings and of the associated rotor winding and each receiving synchro, each synchro causes its shaft 2l to be turned until itV assumes` the same angular orientation asy that of the corresponding transmitting synchro. As will be explained hereinbelow, each receiving synchro acts in the nature of a multi-channel product demodulator, and selects its own particular modulating function while rejecting those of other channels or transmitting synchros, It will also be shown that the resultant action on each receiving synchro is a. steady unidirectional moment which will turn the shaft of each receiving synchro until the torque ceases, leaving each transmitting synchro and its corresponding receiving synchro in alignment at the same angular setting.

In considering the operation of this system, it will be helpful to develop equations expressthe transmitted voltages and received turques in terms which will demonstrate their connection with the theoretical relationships discussed above. This can best be done by nrst examining the dynamics of the receiving synohros. It is evident that the instantaneous moment ma tendingL to move the rotor of the synchro associated with anyl given channel is the snm of two instantaneous moments, one, man, due to the ux associated with the horizontal stator coil, and one, mm, due to the flux associated with the verticalA stator coil. Employi-ng these definitions, we have mn=m7m+mun (19.)

mnnzkmwfmn cos riln, (20) and where k1 is a constant depending on the structure of the synchro, da is the flux generated by current flowing through the rotor, @im is the flux generated by current flowing through the horizontal stator, @vn is the fiux generated by current flowing through the vertical stator, and fr'n is the angle between the rotor and the horizontal stator. In turn, the magnitudes of the stator uxes are given by the equations,

and

where ka is a constant depending on the structure of the synchro, erm is the instantaneous voltage impressed across the horizontal coil, and em is the instantaneous voltage impressed across the vertical coil.

Substituting (20), (2l), (22) and (23) in (19) we have, for the instantaneous total moment tending to move the shaft,

It will be shown later that, if an alternating current operated synchro system be assumed, this equation contains, in addition to a term representing a unidirectional torque, a series of terms representing torques which reverse their directions periodically. It is only the unidirectional torque which tends to move the shaft to a new position; the alternating torques merely cause angular oscillation of the shaft around a mean position determined by the unidirectional torque. If the moment of inertia of the rotor and shaft is sufficiently large, the amplitude of these oscillations is negligibly small. The quantity which controls the rotation of the shaft is therefore not the instantaneous moment m, but the average moment, M, taken over a period of time T equal to the period of the oscillatory term of lowest frequency. This average moment evidently is It is now necessary to consider the transmitting synchros in order to evaluate (br, ehn and cvn. At the transmitting terminal-a master oscillator drives a multivibrator which generates a non-sinusoidal wave, rich inv harmonics, at a fundamental carrier frequency, which, for example, might be 25 cycles per second. This wave is picked up at each transmitting synchro unit from the oscillator bus, and impressed on a synchronized oscillator which is tuned to a harmonic of the fundamental carrier frequency. Harmonics from the sixteenth to the one-hundred and twentieth lying in the range between 400 and 3,000 cycles per second might, for example, be employed in a practical system. In any case, if the fundamental angular frequency is w, the oscillator of any given channel will generate a wave having an angular .frequency nw, where n is the number of the harmonic selected. The output of each oscillator is therefore given by where erm is the instantaneous carrier voltage impressed on the transmitting stators and their associated phasing networks, and Elm is the carrier amplitude.

It is now to be observed that at each transmitting synchro unit, this voltage is impressed on the horizontal stator directly and on the vertical stator through a phase network, which alters the phase of the carrier by degrees. The voltages across the horizontal stator and the vertical stator, assuming negligible losses in the phase network, are therefore Elm cost nwt and Erm sin mot, respectively. These voltages induce corresponding voltages in the rotor coil which are impressed on the first line or radio channel, where their values are respectively equal to yTEkn cos on cos mut and vTEkn sin pn sin nwt, 'YT being a constant depending on line and circuit characteristics and on the construction of the synchro, and on being the angle between the rotor and the horizontal stator. The line voltage due to a single synchro unit, then.

ekn=Ekn cos nuit em=yTEin cos on cos nwt-i-vTEkn sin 4m sin nwt Substituting Een for IyarEim, for the sake of brevity, and observing that each of the transmitting synchros will contribute a voltage to the line in accordance with Equation 27 we have. for the total instantaneous line voltage ei-:KPLEim cos qm cos awt-{- EEOn sin or. sin nwtl (28) It is noted that this equation is identical with Equation l0 defining the transmitted wave in the earlier theoretical discussion. This wave is now transmitted to the receiving terminal, where it is impressed in parallel on the rotors of all the receiving synch-ros. It is this wave which produces the flux I r, generated by current flowing through the rotor of any given receiving synchro. It is seen that if we neglect the line loss, the magnitude of this flux is IJr=k3ei :kslEEon cos qs" cos nwt-l- EEon sin qm sin nwt] (29) where ka is a constant depending on the construction of the synchro.

The funda-mental wave generated by the master oscillator is transmitted over a second line (or radio channel) to the receiving terminal, where it drives a multivibrator, the output of which is impressed in parallel on the inputs of synchronized oscillators associated with the various receiving snychros. These oscillators function in a manner similar to the synchronized oscillators at the transmitting terminal, each producing at its output terminals a wave similar to that dened by Equation 26. In this case, however, the phase network is connected to the horizontal stator rather than to the vertical stator. Neglecting line and circuit losses, the voltages impressed on the receiving horizontal and vertical startors are therefore efm=Ekn sin met (30) and We have now determined the values du, elm, and evn, required for substitution in the equation -`for the averagev moment of the receiving shaft (25).- Making thevsubstitutiomwe obtain "IVI:

TQEO, cosqb cos nwt-l- ZEW, sin sin met] [Ekn sin nwt cos dif-Ek., cos nwt sin pn we now let and choose 'to integrateover'one period of the fundamental carrier wave, which is Y T=21r/1.U the equation becomes t I s M: I l r` M {XEM cos pn cos nwt-l-EM sin 4, sin-nipt] :lig-rein www] 35 The receiving synchro has thus carried out a multi-channel product demodulation operation, in which it has selected its own particular modulating function, and rejected those of other channels. It is seen that in accordance with Equation 35, the result is a steady, uni-directional moment which will turn the shaft until the two angles become equal, when the angle of the sine function becomes (qbvr-qb'n) :0, and the torque ceases, leaving the transmitting and receiving synchros in alignment at the same angle.

It will be understood, of course, that the multi vibrators 25 and 30 can be locally synchronized, thus eliminating the necessity of employing a separate synchronizing pilot line 29. Thus each of the multivibrators 25, 30, can be synchronized by a local tuning fork controlled oscillator or crystal controlled oscillator whose frequency stability is such as to eliminate the necessity of transmitting the synchronizing impulses over a :separate pilot channel.

While in the foregoing, reference has been made to the fact that the rotors I8 of the transmitting synchros or rotary transformers Ill, Il, are controlled manually, it will be understood that they may fbe controlled by respective signal origihating sources represented schematically by the blocks 34, 35, etc. Likewise, if desired, the shafts of the rotors of the receiving synchros l 2, I3, can be connected to respective signal reproducing de vices represented by the blocks 36, 31, etc.

While one particular embodiment has been described herein, it will be understood that various that f changes and modifications may be made therein .l 1. -A .frequency-division without departing from the spirit and scope of channels, means at :each of said originating channels and Aincluding a master oscillatorzto produce thereat a .phase-modulated carrierbut with the` carriers. of respectively different frequenciesand means vto receive and demodulate Vsaid carriers .into their respective signal-reproducing channels, the last-mentioned means comprising a series of receiving synchrosone for each reproducing channel, each synchro having a pair of-` stator windings with the stator windings of ".ea'ch synchro excited by respective carriers of the samerv frequency but displaced in substantial quadrature phase, and means to apply all, the phase-modulated carriers from said transmission channel simultaneously to the rotor windings of said receiving synchros, each of said synchros serving in itself as a frequency selective demodulator for a corresponding one of said phasemodulated carriers.

2. A system according to claim l, in which the means for producing the phase-modulated carrier at each of said originating channels cornprises a transmitting synchro having a pair of stator windings and a rotor winding, and means for energizing the stator windings of each transmitting synchro respectively by two local carrier frequencies of quadrature phase displacement.

3. A system according to claim 1, in which the means at each originating channel for pIOdllCng the respective phase-modulated carriers comprises a corresponding transmitting synchro each synchro having a pair of crossed stator Windings and a cooperating angularly adjustable rotor winding, and means to energize the stator Windings of each transmitting synchro respectively in quadrature phase from a local source of carrier frequency.

4. A frequency-division multiplex system of the type described, a plurality of transmitting synchros each transmitting synchro having an angularly adjustable rotor secondary winding and a pair of crossed primary stator windings, means including a master oscillator to energize the stator windings of each transmitting synchro by respective equal frequency current components which are displaced in phase quadrature with 1'@- spect to each other and with the frequency of each pair different from that of the remaining pairs, a plurality of receiving synchros each having an angularly adjustable rotor primary winding and a pair of crossed secondary stator windings, a signal transmission channel connecting all the rotors of the transmitting synchros in parallel to all the rotors of the receiving synchros, means to energize the stator windings of each receiving synchro by a pair of current components of equal frequency but of relative phase quadrature displacement with the components for one receiving synchro of a different frequency from the components for the remaining receiving synchros, each of said receiving synchros serving in itself as a frequency selective demodulator device for a corresponding one of said frequencies which is used to energize said pairs of stator windings and respective signal devices connected to the rotors of the receiving synchros.

5. A frequency-division multiplex system of 11 the type described, a plurality of signal-originating channels, a master oscillator device for lenerating a series of harmonically-related frequencies, means to supply each originating channel with a particular one of said frequencies and ncluding a corresponding rotary transformer each transformer having a pair of crossed stator Drimary windings and an angularly adjustable rotor secondary winding, means to supply to the stator windings of each transformer but in respective quadrature phase a corresponding one o1 nid channel frequencies, a plurality o! signal-repro ducing channels, a signal transmission channel for interconnecting said originating and reproducing channels, and means to couple said reproducing channels to said transmission channel. the last-mentioned means comprising a plurality of receiving synchros each having an aneularly adjustable rotor winding connected to said transmission channel and a pair ot crossed stator windings coupled to said rotor winding, another device for generating a series of harmonicallyrelated frequencies, and means to supply each pair of stator windings with a particular one of said frequencies but with the excitation of one 12 winding of each pair displaced in quadrature phase with respect to the excitation o! the other winding in each pair, each of said synchros Serving as the sole frequency selection device for a particular one of said frequencies.

DONALD B. HARRIS.

References Cited in the Ille of this patent UNITED STATES PATENTS Number Name Date 1,633,016 Hartley June 21, 1927 1,694,654 Hammond Dec. 11, 1928 2,220,201 Bliss Nov. 5, 1940 2,256,482 Isbister Sept. 23, 1941 2,356,186 Somers Aug. 22, 1944 2,402,973 Moore July 2, 1946 2,407,403 Compton Sept. l0, 1946 2,427,881 Schmitt Sept. 23, 1947 2,534,106 Cohen Dec. l2, 1950 FOREIGN PATENTS Number Country Date 835,470 France Dec. 22, 1938 

